Numerical Modeling of Unstable Water Floods and Tertiary Polymer Floods into Highly Viscous Oils

摘要

The saturation distribution after unstable water flooding into highly viscous oil may have a decisive impact on the efficiency of tertiary polymer flooding, in particular due to hysteresis effects associated with oil banking. In this work, we model water flood and tertiary polymer flood experiments performed on Bentheimer sandstone slabs with heavy oils of about 2000 cP and 7000 cP, and compare the numerical results with experimental production, pressure and X-ray data. The unstable water floods are initially simulated in 2D with our parallel in-house research reservoir simulator using a high-resolution discretization. In agreement with existing literature, we find that Darcy- type simulations based on steady-state relative permeabilities - inferred here from a 3D quasistatic pore network model (PNM) - cannot predict the measured water flood data. Even qualitatively, the viscous fingering patterns are not reproduced. An adaptive dynamic pore network model is then applied on a 2D pore network constructed from the statistics of the 3D network. If the fingering patterns simulated with this 2D PNM are qualitatively in good agreement with the experimental data, a quantitative match cannot be obtained due to the limitations of 2D modeling. Although 3D dynamic PNM at the slab scale would currently lead to prohibitively high computational cost, it has the potential to address the deficiencies of continuum models at highly unfavorable viscosity ratio. For the tertiary polymer floods characterized by a much more favorable mobility ratio, Darcy-type modeling is applied and history matching is conducted from the end of the water floods. We find that unless hysteresis due to oil banking is accounted for in the relative permeability model, it is not possible to reconcile the experimental data sets. This hysteresis phenomenon, associated with oil invasion into previously established water channels, explains the rapid propagation of the oil bank. For the considered experiments, a simultaneous history match of good quality is obtained with the production and pressure data, and the simulated 2D saturation maps are in reasonable agreement with X-ray data. This paper addresses the challenges in modeling highly unstable water flooding, using both a conventional Darcy-type simulator and adaptive dynamic PNM, by confronting the simulated results with experimental data including saturation maps. It also highlights the important role of relative permeability hysteresis in the tertiary recovery of viscous oils by polymer injection.